1. Field of the Invention
This invention relates to a fluorescent display device adapted to display letter, figure or the like in a large size, and more particularly to such a fluorescent display device which allows a casing to be formed of reinforced glass.
2. Description of the Prior Art
A fluorescent display device has been extensively used as a self-luminous type display device for a display panel for a desk calculator, a clock, an electrical appliance such as an audio system or the like, an automobile speedometer, or the like.
Such a conventional fluorescent display device is typically constructed in such a manner as shown in FIGURE 3. More particularly, in FIG. 3 which is an exploded perspective view showing a typical conventional fluorescent display device, reference numeral 1 designates a substrate made of glass or ceramic and formed with through-holes 4a. On the substrate 1 except the through-holes 4a are applied wirings 2 and a light-nonpermeable insulating material 4. On each of the through-holes 4a are laminatedly applied anode conductors 3 and lead terminals 7a.
Then, phosphors 5 respectively exhibiting desired luminous colors are laminated on the anode conductors 3 to form anode sections A. Further, the display device includes filamentary cathodes 8 stretched above the substrate 1 through support members 6 and control electrodes 9 arranged between the filamentary cathodes 8 and the substrate 1. Reference numeral 7 designates lead wires for supplying electricity to each of the electrodes and reference numeral 10 indicates a front cover member which comprises a front plate 10a and side plates 10b each formed of glass or the like and is hermetically bonded to the substrate 1 by means of low-melting frit glass to form a casing B. Reference numeral 11 designates an evacuation tube, through which the casing B is evacuated to high vacuum of about 1.times.10.sup.-5 -1.times.10.sup.-7 Torr. Thereafter, the evacuation tube is sealed to keep the casing at such high vacuum.
In the fluorescent display device constructed as described above, electrons emitted from the filamentary cathodes 8 are accelerated or controlled by the control electrodes 9 and impinged on the phosphors 5 to effect luminous display.
Recently, such a fluorescent display device has been increasingly large-sized in the light of a demand for a display device wherein various kinds of display segments are arranged to carry out complex luminous display, a graphic display device exhibiting multi-display function and the like. Also, in the fluorescent display device, the casing B is constantly applied thereto external force or atmospheric pressure, because the interior is kept at high vacuum as described above. More particularly, supposing that the fluorescent display device has external dimensions as large as, for example, size A4 (210 mm.times.297 mm) and the casing B is formed of sheet glass, a height H.sub.1 of the front plate 10a and that H.sub.3 of the substrate 1 each are required to be as large as about 10 mm in order to provide the casing B with strength sufficient to withstand atmospheric pressure P, as shown in FIG. 4. Further, the arrangement of the filamentary cathodes 8, grid electrodes 9 and the like in the casing B requires to ensure an internal space having a height H.sub.2 of about 5 mm therein. This will cause the overall height H of the casing B to be as large as about 25 mm and the total weight of the front plate 10a and substrate 1 to be as much as about 1.5 kg.
In order to lighten such a problem, it has been proposed and partially practiced to reinforce sheet glass so that it may have sufficient flexural strength in spite of its small thickness. Such reinforcement of the glass has been generally carried out according to the following three processes. One is called a low temperature ion exchange method which is to dip sheet glass in a bath of molten alkali salt containing an alkali ion larger in ionic radius than that contained in the glass at a temperature below the transition point temperature of the glass to carry out the replacement between both alkali ions and then cool the glass to reinforce a surface of the glass due to the difference in volume between both alkali ions replaced. Another is called an air-cooled reinforcement method which is to reinforce sheet glass due to the difference in cooling temperature. The other is called a high temperature ion exchange method which is to replace an alkali ion contained in sheet glass with that having a smaller ionic radius and then cool it to reinforce a surface of the glass due to the difference in expansion coefficient between an interior thereof and the surface. However, only the low temperature ion exchange method has been practiced in view of the strength and deformation of sheet glass reinforced, and the like.
Sheet glass reinforced according to the above-described low temperature ion exchange method (hereinafter referred to as "chemical reinforcement method") has strength about six times as large as ordinary or unreinforced one with respect to flexural stress. However, it has an important disadvantage that the replaced alkali ion is diffused into the glass to cause a decrease in flexural strength.
FIGS. 5(a) and 5(b) each show the relationships between the number of times of a heat treatment repeatedly carried out on reinforced glass under certain conditions and its average breaking flexural stress (an average value of several samples). In FIG. 5(a), the dotted line 51 indicates reinforced glass subjected to a heat treatment at 525.degree. C. for 10 minutes twice, 52 indicates one treated at 560.degree. C. for 7 minutes twice, and the line 53 indicates unreinforced glass. In FIG. 5(b), the line 54 indicates reinforced glass treated at 500.degree. C. for 10 minutes three times and six times, and the chain line 55 indicates one treated at 560.degree. C. for 7 minutes twice and then at 525.degree. C. for 10 minutes twice. As is apparent from FIG. 5, a heat treatment at a high temperature causes strength of reinforced glass to be highly reduced, as indicated by the line 52. However, heat treatment conditions as indicated by the line 54 provides reinforced glass with strength three times as large as sheet glass or more, resulting in it being put into practice.
The application of reinforced glass to the casing of the fluorescent display device will be considered with respect to the substrate 1 and the front plate 10a. The front cover member 10 is subjected to a heat treatment only during the coating of an external electrical field shielding film on the front plate 10a, after the printing of a sealing material on the front plate 10a and side plates 10b, during the assembling between the front plate 10a and the side plate 10b, and during the assembling between the front cover member 10 and the substrate 1. The heat treatments each are generally carried out at a temperature of 480.degree.-520.degree. C. for 5-10 minutes. Accordingly, reinforced glass is permitted to keep strength about 3.5 times as large as sheet glass even after the heat treatment.
Whereas, the substrate 1, when it is one having wirings of a large thickness deposited thereon, is required to be subjected to a heat treatment after the printing of the wirings 2 formed of Ag or the like, after the printing of the light-nonpermeable insulating material 4, after the printing of the anode conductors 4, after the printing of a sealing material on the front cover member 10 and after the printing of phosphors repeated depending upon kinds of the phosphors. The heat treatments are generally carried out at a temperature of 450.degree.-600.degree. C. for about 10 minutes. Accordingly, the use of reinforced glass for the substrate 1 is not significant because the heat treatment reduces its strength to substantially the same degree as unreinforced sheet glass. Also, even if the heat treatment characteristics of reinforced glass is improved to a degree sufficient to keep satisfied strength even after it is subjected to the heat treatment so that the substrate 1 of a substantially small thickness may be formed, it is impossible to avoid the deformation of the substrate 1 unless Young's modulus of the substrate is increased, resulting in non-uniformity of luminance. Thus, the substrate 1 is required to have a significantly large thickness. This causes the fluorescent display device to be hard to be handled during the manufacturing process even when the front cover member 10 is formed of reinforced glass, because the substrate 1 has large thickness and weight as described above. This becomes remarkable particularly for a large-sized fluorescent display device. Further, this has another disadvantage of causing any damage to often occur in the substrate due to cracking or the like during the heat treatment.